1. Field of Invention
The present invention relates to an expandable endoprosthesis that is placed within a tubular member of the human body to treat a region that is pathologically affected by supporting it or holding the tubular member outwards. More specifically ithe invention relates to an intravascular endoprosthesis placed within a blood vessel of the body generally at the site of a vessel lesion in order to provide a more widely open lumen and enhance patency of the vessel. The present invention further relates to a stent that can be used in blood vessels and tubular vessels of the body that have become stenotic or blocked by tissue or other material and require reestablishment of a lumen and maintenance of the lumen.
2. Description of Prior Art
Stents used to internally support tubular vessels of the body can generally be categorized into two groups, those that are mechanically expanded by an external device such as a balloon dilitation catheter, and those that are self-expandable. Advantages of the balloon expandable stents lies in part in the ability of these stents to be delivered accurately to the site of a stenotic lesion. The location of the stent prior to deployment or expansion can be visualized under flouroscopy and deployment of the stent is generally made by inflation of a dilitation balloon which expands the stent radially into contact with the inner surface of the vessel wall. After the dilitation balloon has been deflated and the dilitation catheter removed, the stent is left in place to balance radial forces applied by the vessel wall and ensure that the vessel lumen is maintained in a widely patent conformation.
Some difficulties associated with balloon expandable stents can be related to their lack of flexibility and their inability to withstand external forces that can lead to irreversible crushing of the stent. Several stents exhibit a structure that will not easily bend around tortuous pathways found in the human vasculature to reach the site of the lesion in their nondeployed state. Other stents have a structure that is made more flexible but are not appropriately capable of supporting or balancing the radial forces applied by the vessel acting to compress the stent. A stent with a low radial balancing force characteristics may be expected to undergo an irreversible crushing action if exposed to an externally applied force. Arteries of the neck and leg region can sometimes be exposed to such external forces resulting in permanent deformation of the stent and loss of vessel patency. This has been the case for some balloon expandable stents that have been placed in the carotid artery and exposed to digital or other external forces that have led to collapse of the stent.
In the balloon expandable stents currently found in the prior art one cannot adjust the amount of force required to expand the stent independently from the force required to crush the stent. As a result, a stent design that resists crushing action will generally be too stiff and require too much force to accomplish its deployment.
Self-expandable stents overcome some of the problems associated with the crushability of balloon expanded stents. These stents are typically made of Nitinol, a stainless steel with high yield strength, or some other material that can store energy elastically. Self-expandable stents can be delivered within a sheath to the site of the lesion. There the sheath can be removed and the stent can be deployed as it expands out to a larger diameter associated with its equilibrium diameter.
Some problems associated with self-expandable stents include the inability of the physician delivering the stent to define precisely the location of both ends of the stent. Oftentimes the stent can undergo significant changes in it axial length in going from an nondeployed state to a deployed state. Such length changes can result in inaccuracies in defining a precise placement for the stent. This disadvantage can be somewhat offset by the ability of some self-expandable stents to resist crushing deformation associated with an external force directed toward the side of the stent. Self-expandable stents also do not generally allow the radial expansion force to be adjusted independently from the stent forces that are directed to resist crushing forces. A self-expandable stent with an appropriate elastic balancing force to hold the vessel open may have a weak crush balancing force to resist crushing deformation due to externally applied crushing forces.
Palmaz discloses in U.S. Pat. No. 4,733,665 a balloon-expandable stent that is formed by machining slots into a metal tube forming a series of elongate members and bars. The stent is mounted in its nondeployed state onto the balloon portion of a balloon dilitation catheter and delivered to the site of a lesion that has been previously dilated to allow passage of the stent mounted balloon catheter. Dilation of the balloon causes the balloon-expandable stent to plastically deform at the junction of the elongate members and bars of the stent. For the stent to undergo an expansional deformation the balloon must supply an expansion applied force that exceeds the expansion yield force associated with the junction of the elongate members and the bars. Typically a balloon dilitation catheter used to dilate a coronary lesion found in a three millimeter diameter coronary artery can be dilated at a balloon pressure ranging from one to fifteen atmospheres. With the stent mounted on the balloon, the balloon must be capable of radially expanding the stent and holding the vessel in a widely patent conformation. Upon removal of the balloon catheter, the stent must continue to supply a compression balancing force to balance the compression applied force of the vessel acting inward on the stent. If an externally placed side force is imposed onto the side of the stent, the stent can deform into an oval or flattened shape representative of a crushing deformation. This deformation can involve an elongate member or it can occur at a junction of an elongate member with a bar. The elongate member can be formed such that it resists plastic deformation associated with crushing deformation. The junction of the elongate member with the bar cannot be adjusted to resist crushing deformation without also affecting the force required to expand the stent from its nondeployed state to its deployed state; additionally, the compression balancing force would also be affected. The Palmaz stent disclosed herein therefore can be susceptible to crush deformation in order to maintain appropriate characteristics for an expansion yield force during deployment and a compression yield force to hold the vessel in an open conformation.
A stent is required to have axial flexibility in order to negotiate the tortuous turns found in the coronary vasculature. Palmaz describes in U.S. Pat. No. 5,102,417 connector members that connect between small cylindrical stent segments. Although the connector members provide an enhanced axial flexibility, this stent is still subject to crush deformation. The compression yield force that is capable of holding the vessel outward with the stent in a deployed state is coupled to the crush yield force that prevents the stent from crush deformation.
Fischell describes in U.S. Pat. No. 5,695,516 a balloon-expandable stent formed from a metal tube and having circumferential arcs and diagonal struts. When this stent is expanded to a deployed state the junctions of the arcs and struts undergo plastic deformation and the deployed stent takes on a honeycomb shape. The expansion yield force of this stent describes the force required to plastically deform an arc with respect to a strut at a junction during stent expansion. The compression yield force describes the force required to plastically deform an arc with respect to a strut when exposed to a compression applied force by the blood vessel. Deformation due to crushing would also occur at the junction of the arc with the strut. The crush yield force of this stent is therefore directly coupled to the expansion yield force and the compression yield force. If this stent were designed to expand upon exposure to a dilitation balloon with a nominal expansion applied force, it would not be able to resist crushing deformation when exposed to an external side force that can be encountered in a carotid or femoral artery position.
Fischell describes in U.S. Pat. No. 5,679,971 a balloon-expandable stent that has two different types of cells, one for radial rigidity and one for axial flexibility. Neither one of these cells addresses the need to provide an anti-crush characteristic to the stent. One cannot adjust the crush yield strength of this stent independently of the expansion yield strength.
Lam (U.S. Pat. No. 5,649,952), Anderson (U.S. Pat. No. 5,800,526), Frantzen (U.S. Pat. No. 5,843,164), and Orth (U.S. Pat. No. 5,681,346) each describe balloon-expandable stents formed from metal cylinders and having serpentine wave patterns connected by interconnecting members. In each of these stent disclosures the stent has a crush yield force that is coupled to the expansion and compression yield force. These stents would be susceptible to irreversible crush deformation if exposed to an external force to the side of the stent.
Gianturco discloses in U.S. Pat. No. 4,800,882 a balloon-expandable stent constructed from a metal wire and discloses in U.S. Pat. No. 5,041,126 a method of insertion for a balloon-expandable stent. The wire stent disclosed by Gianturco has adjacent curved sections or loops joined by a bend or cusp. During stent expansion by a balloon the loops diverge as the metal wire irreversibly and plastically deforms. The deformation of the metal wire that occurs during expansion has a similar yield force as the deformation of the metal wire that can occur during compression of the stent if the compression applied force of the vessel exceeds the compression yield strength of the stent. If this stent is exposed to an external side force that could lead to a crush deformation, this stent can undergo plastic deformation that is irreversible and can occur with a similar yield force as the compression yield force. This stent is not well suited to provide independent adjustment of compression yield force with respect to crush yield force. As a result, this stent can be subject to crushing deformation even though the expansion and compression yield force are appropriate for allowing balloon expansion and support of the blood vessel.
Hillstead (U.S. Pat. No. 4,856,516), Wiktor (U.S. Pat. Nos. 5,133,732 and 4,886,062), Globerman (U.S. Pat. No. 5,776,161), Fontaine (U.S. Pat. No. 5,527,354), Horn (U.S. Pat. No. 5,591,230), Boyle (U.S. Pat. Nos. 5,613,981 and 5,591,198), and Hillstead (U.S. Pat. No. 5,116,365) each describe balloon-expandable wire stents made from loops, zig zags, helical wires, curved rings, sinusoidal waves, or other similar form of construction. All of the stents described in these disclosures are expandable by the expansion applied forces of a balloon of a balloon dilitation catheter or other similar catheter. During the expansion of the stents, the wires or each stent undergoes a plastic deformation once the expansion yield force has been exceeded. A plastic deformation would also be required for any of these stents to compress under the compression applied force applied by the vessel wall; this could occur once the compression yield force of the stent has been exceeded. Exposure of any of these stents to an external side force could lead to a crush deformation. It is not possible for any of these stents to enhance the crush yield force without altering the expansion or compression yield force of the stent. Cox (U.S. Pat. No. 5,733,330) and Wall (U.S. Pat. No. 5,192,307) each describe balloon-expandable stents with ratchet mechanisms. The stents can be formed out of an elastic metal that will not allow for stent crushing and the ratchet mechanism can prevent the stents from collapsing under the force of compression applied by the blood vessel. Each one of these disclosures describes a separate latching or ratcheting mechanism that is required to provide the properties of balloon-expandable and non-crushability. The latching or ratcheting mechanism adds to the size and the complexity of the device.
Wallsten describes in U.S. Pat. Nos. 4,655,771 and 5,061,275 self-expandable stents formed from helically wound braided flexible thread elements or wires. The metal wires are elastic or resilient in nature with a high energy storage capacity. The stents can be delivered to the site of the lesion by an external sheath that applies a constraining force upon the stent to hold it in an nondeployed state of a smaller diameter. Upon release of the stent within the blood vessel, the stent undergoes a radial expansion to a larger predetermined diameter. The vessel provides a compression applied force due to vessel elasticity and collagenous scarring and contraction that can occur during vessel healing; this compression applied force acts inward on the stent. The stent in its deployed state provides an expansion elastic balancing force outward against the vessel wall to balance the compression applied force. If the stents described by these disclosures are acted upon by an external crush applied force delivered to the side of the stent, the stents can undergo an crush deformation forming an oval or flattened shape. The stents can provide a crush elastic balancing force to balance the crush applied force and limit the amount of crush deformation. The degree of crush deformation that can occur for a specific crush applied force is directly related to the size and number of flexible thread elements or wires used in the formation of the stents. The size and number of wires has a direct bearing on the expansion elastic balancing force provided by the stents. Therefore an increase in the crush balancing force will generally be associated with a corresponding increase in elastic balancing force. It can be desirable to adjust the crush balancing force independently of the elastic balancing force. The stents disclosed by Wallsten are not well suited to independent adjustment of the crush and the expansion elastic balancing force. Similarly, the crush balancing force is directly coupled to the expansion elastic balancing force.
Gianturco describes in U.S. Pat. Nos. 4,580,568 and 5,035,706 self-expandable stents formed from metal wire in a zig-zag pattern. These stents are elastically compressed to a smaller diameter for delivery within a blood vessel and undergo an elastic expansional deformation during delivery at the lesion site. The stents provide a deployed expansion elastic balancing force outward against the vessel wall to maintain the diameter of the vessel in an open and widely patent conformation. If exposed to an external crush applied force the stents will deform elastically and provide a crush elastic balancing force. The expansion and crush elastic balancing force are directly coupled and are not easily varied with respect to one another. Such stents with appropriate expansion characteristics are not easily adjusted to provide altered crush characteristics independently of one another.
Lauterjung (U.S. Pat. No. 5,630,829) and An (U.S. Pat. No. 5,545,211) describe self-expandable stents formed from metal wire. Lauterjung provides a high hoop strength stent due to the angle of the wire in the expanded state. An describes a zig-zag pattern that is spiraled into turns and is cross-linked with each other at adjacent turns. Each of these stents has an expansion and a crush elastic balancing force that is coupled directly to each other. One can not appropriately adjust the expansion characteristic, with respect to the crush characteristics.
Carpenter describes in U.S. Pat. No. 5,643,314 made of a series of elastic metal bands or loops interconnected along a backbone. A lock is used to hold the loops in a contracted configuration around a balloon portion of a delivery catheter during delivery to the lesion site. Once expanded by the balloon, a lock is used to hold the loops outward in their expanded configuration. The strength of the lock provides the balancing force of the stent to hold the vessel in an open widely patent configuration. The dimensions of the metal loops determines the crush balancing force for a specific crush deformation. This stent is cumbersome to use with sliding required between metal and a locking mechanism that occupies areal space and volume.
McIntyre (U.S. Pat. No. 5,833,707) and McDonald (U.S. Pat. No. 5,728,150) each describe a stent formed from a flexible elastic metal sheet that has been coiled into a small diameter for delivery to a blood vessel. Upon release of the coiled stent, it springs out to form a larger deployed diameter and hold the vessel with an expansion elastic balancing force. The crush balancing force of these stents involves a similar defomation of the metal sheet as the expansion or compression deformation involved with the expansion delivery of the stents or collapse of the stents due to vessel compression applied forces. These stents do not provide for adjustment of the crush balancing force with respect to the expansion or compression balancing force.
Dotter (U.S. Pat. No. 4,503,569), Alfidi (U.S. Pat. No. 3,868,956), and Froix (U.S. Pat. No. 5,607,467) describe coiled stents constructed out of plastic or metal that can change in shape from a small diameter to a large diameter due to the application of heat, or application of another external condition. Most plastic stents are not acceptable due to the inadequate strength per volume of material in comparison to a metal stent. As a result, plastic stents require excessive areal space or volume which can be very undesirable in a small diameter blood vessel. Metal stents with a coiled shape have a similar mode of deformation in providing an expansion or compression balancing force in comparison to providing a crush balancing force. These stents do not allow the crush balancing force to be adjusted with respect to the expansion or compression balancing force.
Roubin discloses in U.S. Pat. No. 5,827,321 a stent that is radially expandable by balloon or self-expandable and designed to maintain its axial length upon expansion. The stent has annular elements connected by connecting members. The connecting members are formed from Nitinol and have a desire to lengthen upon deployment of the stent. The expansion or compression balancing force against the vessel wall is provided by the annular elements which have a curved or zig-zag structure. Upon exposure to a crush deformation it is the annular elements that provide the crush balancing force. The crush balancing force is coupled to the expansion or compression balancing force; the stent does not provide for independent adjustment of the balancing forces.
Williams (U.S. Pat. No. 5,827,322) describes a balloon-expandable or self-expandable stent formed from Nitinol flat metal sheet and having a ratchet mechanism to hold the stent in an expanded state. This stent is not flexible in the axial direction and the ratchet mechanism requires additional areal space and volume. This stent does not allow independent adjustment of crush balancing force without also significantly impacting the expansion elastic balancing force provided by the stent.
Hilaire describes in International Application with International Publication Number WO 98/58600 an expandable stent with variable thickness. The variable thickness is intended to allow the balloon expandable stent to expand more evenly along its perimeter. The stent is formed from a plurality of tubular elements with a zig zag shape that are joined together by linking members. This device does not teach or describe a stent that provides independent adjustment of stent expansion forces with respect to stent crush forces.